A tank-venting valve is an example of a flow valve. A tank-venting valve is used as a component of a tank-venting apparatus for a motor vehicle driven by an internal combustion engine. Tank-venting apparatus function to collect fuel vapors, to store these vapors and to supply the vapors to the engine for combustion. The fuel vapors are formed in a supply tank by vaporization during tanking and/or operation of the motor vehicle. A defective tank-venting valve can reduce the quality of the exhaust gas of the internal combustion engine. Statutory requirements exist in accordance with which the tank-venting valve is monitored during operation of the internal combustion engine (on-board diagnostics).
U.S. Pat. No. 5,243,853 discloses a system for monitoring the operability of a tank-venting valve based on an internal combustion engine having a controlled idle rpm and a controlled air/fuel mixture ratio (.lambda.). This state of the art provides that the tank-venting valve is opened during idle of the engine and the reactions of the lambda control or of the idle control are evaluated for diagnosis. These reactions are different in dependence upon the composition of the vapor flowing through the open tank-venting valve. If, in an extreme case, the gas consists of pure air or, in another extreme case, the gas is pure fuel vapor, then a corresponding compensating enrichment or leaning takes place via the lambda control. If the enrichment or leaning exceeds a predetermined threshold value, then the tank-venting valve is considered to be in order.
If the composition of the gas corresponds to the desired value (such as .lambda.=1) wanted for the operation of the engine, then the lambda controller does not carry out a correction. The influence of the additional mixture quantity would then lead to an increased idle rpm in the absence of a countering actuating intervention by the idle controller. This additional mixture quantity results when the opening cross sections of the tank-venting valve are large. The tank-venting valve is in order when a reaction of the idle controller occurs which exceeds a predetermined threshold value. If in contrast, this threshold value is not exceeded, then the tank-venting valve is deemed to be defective.
It has been shown that the reliability of the diagnostic data of the described system is reduced under certain conditions such as during operation of the motor vehicle at high elevations above sea level or in combination with driving ancillary equipment during idle of the engine.
To avoid incorrect data, it is known in this context, for example, to monitor the load of the engine during the diagnosis and to interrupt the diagnosis when load changes occur. During idle of the internal combustion engine, such load changes are caused, for example, by: actuation of the power steering, switching in electrical consumers or engaging a drive gear in a vehicle having an automatic transmission. It has further been shown that the reliability of the diagnostic data is also reduced with increasing load during idle so that even for constant increased load (such as caused by a climate control which is switched in), the problem is present that incorrect messages occur during the diagnosis. Blanking out the diagnosis in such operating states can therefore lead to a situation (especially in summer) that an existing defect remains unnoticed longer than necessary.